The invention relates to a valve arrangement for controlling hydraulic fluid in an axial piston machine in which a first flow duct is provided in a stationary part and opens out into a second flow duct running transversely, an accommodating space of the stationary part being formed lying opposite the second flow duct in the extension of the first flow duct and being intended for accommodating and guiding a multiple valve body, which can be displaced in the direction of the central axis of the flow duct and in which the multiple valve body comprises a first and a second valve body which are arranged one behind the other in the direction of the longitudinal axis of the first flow duct and can be displaced in relation to each other in this direction.
A valve arrangement of this type has frequently been fitted in various axial piston machines. The valve PLC 182 on page 6–9–3 of publication HY17-8702/UK of February 2001 by Parker Mobile Hydraulics is one such example. In this case, the first flow duct is connected to a hydraulic fluid store which is under low pressure, i.e. it serves as a feed-in line. The second flow duct of the known valve arrangement serves as a connecting line to a high-pressure working line, and can therefore be under high pressure or low pressure depending on the operating conditions. The two valve bodies, together with associated valve seats form a first and a second valve, one serving as a feed valve and enabling hydraulic fluid which is under low pressure to flow from the first flow duct into the second flow duct. By contrast, the other valve serves as a pressure-limiting valve and enables hydraulic fluid to flow from the connecting line into the feed line if the working pressure in the high-pressure working line has become impermissibly high.
The known valve arrangement is therefore a directly controlled, preset high-pressure limiting valve with a feed-in function. In terms of structure, it is designed in such a manner that the first valve body is inserted into the second valve body where it has its valve seat, while the second valve body interacts with a seat formed in the stationary part. Two valves are therefore fitted one inside the other, the two valve seats being spaced apart only a little distance from each other in the direction of the central axis of the first flow duct. In this known valve arrangement, the through flow cross section is therefore restricted for structural reasons. This is true with a pressure-limiting valve because the valve serving for this purpose has to be arranged within the cross section of the valve body that implements the feed-in function.
However, it is precisely for the pressure-limiting function that a rapid response is imperative, for which purpose relatively large flow cross sections are desired. In order to satisfy this demand, the known valve arrangement would have to be designed with very large cross sections. However, this is not possible when it is fitted into axial piston machines.
Pilot-controlled valves are therefore generally used in cases of this type. One example of this is illustrated and described in DE 102 39 725 A1. According to this, a hydraulic pressure-limiting valve has a first flow duct and a second flow duct running transversely with respect to it. This pressure-limiting valve permits hydraulic fluid to pass from the first flow duct into the second flow duct if the pressure of the hydraulic fluid in the first flow duct exceeds a certain threshold value. For this purpose, a first valve body is arranged in the region between the two flow ducts. The first valve body is bored through, so that the first flow duct is connected continuously to the space behind the first valve body. A second valve is arranged behind the first valve body, coaxially with it, and serves as a pilot valve. If the pressure of the hydraulic fluid in the first flow duct exceeds a threshold value, the pilot valve opens. In conjunction with a correctly dimensioned restoring spring, the effect achieved by the dropping pressure on the rear side of the first valve body is that the opening and closing forces for the first valve body are reduced. However, the larger a valve is, the more expensive it is to produce and to use. Pilot-controlled valves are always more expensive than directly controlled valves. In addition, they have to be designed such that they can be adjusted because they cannot be preset with sufficient accuracy.
It is therefore a primary object of the present invention to provide a valve arrangement of the type mentioned, which permits large throughflows with a low pressure loss, takes up little installation space and can be produced cost-effectively.
These and other objects will be apparent to those skilled in the art.